Effects of inoculum concentration, leaf age and wetness period on the development of dark leaf and pod spot (Alternaria brassicae ) on oilseed rape (Brassica napus )

Experiments were done under controlled environment and glasshouse conditions to study the effects of inoculum concentration, leaf age and wetness period on the development of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus). On leaves of potted oilseed rape plants (cv. Bienvenu) inoculated with A. brassicae conidial suspensions, the severity (number of lesions cm(-2)) of dark leaf spot increased as inoculum concentration increased from 80 to 660 Spores ml(-1) and as leaf age increased from 4 to 14 days. On pods on detached racemes of spring oilseed rape (cv. Starlight), the incidence of dark pod spot (% of pods diseased) increased as inoculum concentration increased from 80 to 10(4) spores ml(-1). Increasing inoculum concentration above 10(4) spores ml(-1) did not increase the incidence but did increase the severity of dark pod spot. A minimum wetness period of 4 h was needed for infection of oilseed rape leaves (cv. Envol) by A. brassicae at 18 degrees C and disease severity increased with increasing wetness period up to 12 h. The length of dry interruptions after 3-8 h of initial wetness affected the severity of dark leaf spot. A second wetness period increased the severity of dark leaf spot if the dry interruption was less than or equal to 6 h and if the first wetness period was less than or equal to 8 h. The incubation period of A. brassicae decreased from 3.5 to 2.5 days as inoculum concentration increased from 80 to 660 spores ml(-1) on leaves (cv. Bienvenu) at 17-25 degrees C and from 3.8 to 1.0 day as inoculum concentration increased from 80 to greater than or equal to 2 x 10(3) spores ml(-1) on pods (cv. Starlight) at 18 degrees C.